This invention relates to surface acoustic wave (SAW) devices having a plurality of closely spaced metallic thin film structures disposed in close proximity to one another on a substrate surface.
A persistent problem encountered in the fabrication of SAW devices having a plurality of closely spaced metallic thin film structures disposed in close proximity to one another is the formation of surface cracks in the piezoelectric substrate, normally lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3) between such closely spaced metallic thin films. these cracks almost always develop during or shortly after the fabrication of the SAW device known as a SAW elastic convolver. The SAW elastic convolver, which directly utilizes the acoustic non-linearities of a piezoelectric substrate, comprises a set of opposing parabolically tapered horns to reduce the acoustic beamwidth of waves incident thereon and which are coupled thereto from acoustic wave generating transducers. The output from each horn, which is at the horn throat, is coupled to a narrow interaction channel where the actual signal convolution is accomplished. The beam compression provided by the horns increases the acoustic power density in the interactive channel so as to increase the convolver efficiency. The convolver elements described above are generally in the form of microstrip deposited on the piezoelectric substrate. The above mentioned coupling of the beam compressed waves from the tapered horn to the interactive channel is on the surface of the piezoelectric substrate across a narrow gap in the microstrip, between the horn throat and the interactive channel. The above mentioned cracks in the surface of the piezoelectric substrate almost always occur in these gaps at the final photolithographic lift-off process used to define a microstrip pattern on the substrate, although the cracks may not develop for several hours after the lift-off is completed. Further, the position of the cracks appears to be crystal orientation sensitive in that the cracks observed to data always occur at either the left or right hand edges of the gaps (there are at least two gaps on each substrate of a convolver) but never at both the left and right hand gap edges.
The presence of such cracks degrades the performance of SAW devices in that the cracks act as a reflection boundary to surface acoustic waves propagating along the surface of the piezoelectric substrate between the microstrip patterns. In SAW convolvers, in particular, such cracks in the gap between a tapered horn and the interaction channel cause a portion of the surface acoustic signals traveling thereover to be reflected back from the cracks. Such reflected signals interact (convolve) with the forward moving signals creating a self-convolution spurious signal which limits the device signal-to-noise ratio and the effective dynamic range.
The formation of such cracks is believed to be caused by electric fields developed across the gaps during fabrication of the SAW device. The source of such electric fields has been difficult to determine, however, it has been speculated that the electric fields are caused by static pickup, pyroelectric effects, trapped charges, stress or a combination of these.